1. Field of the Invention
The present invention relates to a method and apparatus for inspecting an optical information storage medium, an optical information storage medium, and a method of writing information on such a medium. More particularly, the present invention relates to a method and apparatus for inspecting an optical information storage medium on which a read/write operation needs to be performed at high rates and also relates to a method for inspecting such an optical information storage medium for residual focus and tracking errors.
2. Description of the Related Art
An optical information storage medium has a storage layer on which information is written as pits or marks. That information can be read by irradiating the pits or marks with light and by detecting a variation in the intensity of the light reflected. Such an optical information storage medium normally has a disc shape, and therefore, is called an “optical disc”. Thus, according to that normal practice, an optical information storage medium will be simply referred to herein as an “optical disc”.
Nowadays, Blu-ray discs (BDs), digital versatile discs (DVDs) and other optical discs with high densities and big storage capacities have become increasing popular and have been used more and more extensively to store computer data, software, audiovisual data and so on.
Among those optical discs with high densities and big storage capacities, there are increasing demands on the market for write-once discs such as DVD-Rs and BD-Rs, in particular. A write-once optical disc may have a storage layer including a Te—O—M based material (where M is at least one element selected from the group consisting of metallic elements, metalloid elements and semiconductor elements) as disclosed in Japanese Patent Application Laid-Open Publication No. 2004-362748. The Te—O—M based material is a compound material, which includes Te, O and M and in which fine particles of Te, Te—M and M are randomly dispersed in a TeO2 matrix of the as-deposited material. When the storage layer of such a material is irradiated with a laser beam with at least a predetermined intensity, the portion of the storage layer irradiated with the laser beam will melt to precipitate Te or Te—M crystals with large particle sizes while being cooling, thereby forming a recording mark on the storage layer. That portion where the crystals have been precipitated has a different optical property from the other portions. That is why when the recording mark is irradiated with a laser beam, a difference will be made on the intensity of the reflected light and the difference in the intensity of the reflected light can be detected as a signal. In this manner, a so-called “write-once operation”, which allows the user to perform a write operation only once, can get done.
The rotational velocity of an optical disc can be controlled by a CLV (constant linear velocity) technique or a CAV (constant angular velocity) technique. Specifically, according to the CLV control technique, the rotational frequency of a given optical disc is controlled inversely proportional to the radial location and information is supposed to be written in response to a certain number of write channel clock pulses while making the scanning light beam follow the tracks at a constant linear velocity. On the other hand, according to the CAV control technique, the rotational frequency is kept constant even while data is being written on the optical disc but channel clock pulses are applied during writing as a reference signal to the optical disc at variable frequencies that are proportional to the radial location of the scanning light beam on the tracks. In that case, channel clock pulses are applied at low frequencies on the inside portion of the disc but are applied at high frequencies on the outside portion of the disc. Then the recording linear velocity will be low on the inside portion and high on the outside portion but recording marks will be left with a constant recording linear density.
In writing information on an optical disc or reading the information stored there from the disc, the optical disc needs to be irradiated with a laser beam that has been converged in a predetermined state. In such a situation, a type of control to be performed by an optical disc drive to keep the laser beam in such a predetermined converged state is called a “focus servo control”, while another type of control to be performed by the optical disc drive to move the laser beam spot in the disc radial direction so as to follow the tracks, which are a series of marks left on the storage layer, is called a “tracking servo control”. Also, a signal representing the magnitude of shift from the predetermined converged state of the laser beam in the focus servo control is called a “focus error signal”. Likewise, a signal representing the magnitude of deviation of the laser beam from the target tracks in the tracking servo control is called a “tracking error signal”. The tracking error is sometimes called a “radial tracking error” and the focus error is sometimes called an “axial tracking error”.
For example, Japanese Patent Application Laid-Open Publication No. 2004-5817 and Japanese Patent Publication No. 3819138 disclose technologies relating to focus and tracking servo controls to be performed on a write-once optical disc. These documents disclose an optical disc drive and method for performing write processing with high reliability by controlling the write rate based on the focus error signal and other signals and a method for detecting the values of vibrations to be produced due to the eccentricity of the disc based on the tracking error signal.
Recently, particularly in computer peripheral devices and optical disc recorders that are compatible with optical discs with huge storage capacities, it is more necessary to get a write operation done at high transfer rates than anything else. Specifically, there is an increasing demand for development of a technique for reading or writing information at rates corresponding to 6× velocity for BDs. To achieve such high transfer rates, however, the optical disc should be scanned with a laser beam much more quickly by increasing the rotational frequency (or the linear velocity) of the disc. As used herein, the “**× velocity”, for example, means that the velocity is ** times as high as the standard read/write rate. More specifically, the read/write rate is represented as either a linear velocity or a transfer rate. In this description, the read/write rate will be represented herein by the linear velocity in most cases.
Generally speaking, however, if the rotational frequency of a disc were increased, then the locations on the tracks where the information is written and the levels (i.e., heights) of the storage layer would change quickly due to out-of-plane vibrations, eccentricity, defects, variations in thickness distribution and other shape imperfections of the optical disc. Thus, the focus servo control and the tracking servo control should be performed even more quickly. However, there is a certain limit to the response of the servo control. That is why if the on-track locations or the levels of the storage layer changed at frequencies that are even higher than the quickest possible response of the servo control, then it would be impossible for the optical disc drive to get the focus servo control or the tracking servo control done perfectly. As a result, the tracking error signal would have an increased residual error (which will be referred to herein as a “residual tracking error”), thus decreasing the stability of the tracking servo. And the residual error of a focus error signal (which will be referred to herein as a “residual focus error”) would also increase and the envelope of a write signal would have missing (or zero-amplitude) portions corresponding to the residual error to possibly decrease the symbol error rate (SER) significantly.
As used herein, the “residual tracking error” refers to a signal component to be produced in a situation where the tracking control has not been done quite successfully. That is to say, even if the optical disc drive is performing a tracking servo control appropriately enough, the laser beam may still be unable to follow the tracks perfectly to make the level of the tracking error signal not equal to zero, which is what is called a “residual tracking error”. Likewise, the “residual focus error” refers to a signal component to be produced in a situation where the focus control has not been done quite successfully. That is to say, even if the optical disc drive is performing a focus servo control appropriately enough, the laser beam may still deviate from the predetermined converged state to make the level of the focus error signal not equal to zero, which is what is called “residual focus error”. The residual error of each of these signals is estimated by the amplitude of that signal. And the optical disc drive represents the values of those residual errors by the magnitude of deviation of the laser beam spot from the center of the tracks and by that of shift of the focal point of the laser beam from the target storage layer, respectively. More specifically, these magnitudes are represented as distances (or lengths). That is why the tracking error signal may be represented as having a residual error of xx nm and the focus error signal may be represented as having a residual error of xx nm. It should be noted that the residual errors are sometimes called simply “residuals”. In this description, when just “residual errors” are mentioned, the residual errors refer to both the residual tracking error and the residual focus error alike.
For these reasons, it is necessary to control the shape of a stamper to be used as a master to make an optical disc, the forming process of the optical disc, the viscosity of the resin material of its coating layer, and the thickness of a spin-coated film with even higher degrees of precision. Added to that, it is no less important to develop an inspecting method and apparatus that can efficiently and precisely determine whether or not the optical disc product just made has expected shape precision or mechanical properties.
However, if the spindle motor of such an inspecting apparatus carried out the inspection while rotating at six times as high velocities as normal BDs, then significant residual focus and tracking errors would be detected from mechanical factors of the inspecting apparatus itself, e.g., vibrations and resonance of the actuator. Then, it would be impossible to precisely measure the residual errors that have been caused due to the mechanical properties of the optical disc (or get the inspection done) just as originally intended. Nevertheless, if an expensive high-performance inspecting apparatus that would have reduced vibrations or actuator resonance were newly introduced, then investment on equipment should be newly made, thus eventually increasing the manufacturing cost of the media.
Also, if a write operation were performed by the CLV control technique on the entire surface of an optical disc at as high a linear velocity as 6× rate for BDs, then the rotational frequency of the spindle motor should be higher than 10,000 rpm on the inside portion of the disc. This is a problem because 10,000 rpm is the maximum allowable rotational frequency in practice that was determined from safety considerations in view of the rupture limit of plastic that is the substrate material of the disc. For that reason, the optical disc should not be inspected at such a high velocity as exceeding 10,000 rpm.
Furthermore, the residual errors of the tracking error signal or the focus error signal could be reduced by performing the servo controls with higher precision with the servo filter characteristic of the inspecting apparatus adjusted. However, an optical disc drive that performs a write operation on BDs at 4× linear velocity performs focus and tracking servo control operations using a servo filter that already has as high a gain intersection as 6 kHz to 8 kHz. For that reason, if the servo characteristic of the inspecting apparatus should have an even higher gain intersection to cope with the 6× linear velocity for BDs, then the actuator would have a decreased oscillation or phase margin, thus making it virtually impossible to secure servo stability.
In order to overcome the problems described above, the present invention has an object of providing a method and apparatus for precisely inspecting an optical information storage medium, on which a read/write operation should be performed at high linear velocities. Another object of the present invention is to provide a method of writing a signal of quality on such an optical information storage medium. Still another object of the present invention is to provide such an optical information storage medium.